C-4 Filament Design Miniature Light Bulbs have a single coil (C) filament design. Unless noted otherwise, C-4 filament design light bulbs utilize tungsten filament construction because of its (tungsten) high melting point at incandescence. The number following the coil design indicator (C) indicates the design of the filament supports. The filament is the primary light producing component in a light bulb. Its design is critical to ensuring the lamp will operate as intended. Unless specified otherwise, lamp filaments today are made of tungsten, in various shapes/designs for different applications. Tungsten filaments have replaced carbon filaments due to their increased durability and because carbon filaments evaporate more rapidly at high temperatures whereas tungsten filaments have slower rates of evaporation at high temperatures.